Ink jet printers are well known and widely used. Such printers typically incorporate a scanning carriage which supports one or more inkjet printheads. Thermal inkjet printheads operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of plural orifices or nozzles so as to print a dot of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays on a nozzle plate. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed on the paper as the printhead is moved relative to the paper. One type of thermal printhead is described in U.S. Pat. No. 5,278,584 assigned to the present assignee and incorporated herein by reference.
A related type of ink jet printer uses piezo-electric elements, instead of heaters, to eject ink from an associated orifice. The present invention applies to both thermal and nonthermal ink jet printers.
One drawback of inkjet printers is that the ink ejection elements, whether heater resistors or piezoelectric elements, and their associated nozzles, wear unevenly due to nonuniform use of the various ink ejection elements and nozzles in the scanning printhead. This nonuniform use, or "nozzle affinity," can be measured over the course of printer output (e.g. a page, a document, etc.) and is expressed as a ratio. Nozzle affinity is the ratio of the number of dots printed by the most active nozzle, to the average number of dots printed by all the nozzles. Thus, if one nozzle prints 4800 dots in a sample output, while the average nozzle prints 1200 dots, the nozzle affinity is 4.0. Nozzle affinities in the prior art generally range from about 2.0 to 6.0, with the greatest imbalances being associated with documents of regularly-spaced text.
Although a printhead may have hundreds of nozzles, replacement is usually required when one nozzle (or a few) fails. Accordingly, it will be recognized that the useful lifetime of a printhead can be extended considerably (e.g. by factors or two or more) if the nozzle affinity ratio can be reduced. So doing reduces wear on the most active nozzle--the nozzle most prone to failure.
A prior art approach to reducing nozzle affinity is disclosed in a copending application by Nobel et al, Ser. No. 08/490,268, filed Jun. 14, 1995, entitled Inkjet Printing With Uniform Wear of Ink Ejection Elements and Nozzles, assigned to the present assignee and incorporated by reference. Referring to FIG. 1, Nobel's preferred embodiment does not consistently align the top 12 of the printhead 14 with the top of a swath of text to be printed, as was conventionally done in the prior art (FIG. 1 at `A`). Instead, Nobel et al sometimes aligns the bottom 16 of the printhead with the bottom of the swath of text so as to even out the nozzle usage (FIG. 1 at `B`). This arrangement causes nozzles that previously aligned with spaces between text lines to sometimes align with printable text, so as to even out nozzle usage. However, significant usage disparities can still exist (e.g. in FIG. 1 example, alternating between printhead placements A and B still leaves a quarter of the printhead nozzles unused, namely at areas 18 and 20.) Nobel et al discloses variant nozzle balancing techniques as well.
The present invention extends and improves the nozzle balancing techniques disclosed by Nobel et al.
The following disclosure proceeds with reference to an exemplary single-color (e.g. black ink) printhead having 304 nozzles, arrayed in two staggered columns of 152 nozzles each. The vertical spacing between nozzles in each column is 1/300.sup.th of an inch. The vertical spacing between successive nozzles in the staggered columns is thus 1/600.sup.th of an inch. (It will be recognized that the foregoing printhead is exemplary only. The invention can similarly be practiced with color printheads, multi-color printheads, printers employing plural distinct printheads, etc.) For purposes of the present disclosure, the printhead nozzles are numbered consecutively beginning at the top, with the odd-numbered nozzles in one column, and the even-numbered nozzles in the other.
The present disclosure sometimes refers to the printhead travelling down the piece of paper, from top to bottom. Of course, in most printers the printhead moves only laterally, and the paper moves in the other dimension. However, conceptualization of a raster-scanning printhead that moves both down and across the paper is sometimes convenient.
By way of further background, it is helpful to detail certain nuances of printer operation that may be taken into account in addressing the nozzle affinity problem.
In the prior art, certain Hewlett-Packard Co. printers have employed multi-pass printing techniques to increase print quality when several passes of the printhead over a swath are required. (Some high quality printing modes entail eight passes.) In particular, such multi-pass algorithms seek to print successive passes using different groups of printhead nozzles. For example, in a high quality black text print mode where two passes over a swath of text are required, the first pass may employ nozzles 101-200, and--after moving the printhead down slightly--the second pass may employ nozzles 1-100. By this arrangement, any nozzle irregularities (e.g. light or no output) are masked by subsequent passes, rather than compounded.
Another technique for optimizing print quality is to avoid printing fractional lines of text (i.e. tops of characters in one pass; bottoms on next pass). If a line is split, columns of dots from one pass may not precisely align with columns of dots from the second pass, causing a discontinuity, or "jitter," in the characters, mid-line.
The foregoing problem of apparent discontinuities at vertical swath boundaries cannot be avoided when printing graphics (or text) taller than the printhead. To mitigate such discontinuities, however, the normal bidirectional mode of printing (printing during both the left-to-right and right-to-left movements of the printhead) is commonly suspended, and unidirectional printing is employed instead. Unidirectional printing provides more consistent droplet placement--swath to swath--than bidirectional printing.
Discontinuity errors that still persist with unidirectional printing can be obscured by use of multi-pass printing techniques. Consider the printing of a graphical image that is 2 printheads (i.e. 608 droplets) in height, and requires four passes of the printhead over each swath. One approach is to use the full height of the printhead four times along one swath, advance the paper once, and then use the full height of the printhead four times at the adjoining swath. Such action effectively reforms the same boundary four times, emphasizing same and increasing its visibility.
Printers using HP's multi-pass print techniques proceed differently. An exemplary printing procedure might divide the 608 lines of droplets into eight fractional swaths A-H, which are printed in eleven successive unidirectional passes, with the paper advanced between each, as shown in FIG. 2. Each fractional swath is printed four times (e.g. swath B is printed in passes 2, 3, 4 and 5). By this arrangement, any discontinuity introduced by column misalignment where the top and bottom edges of the printhead adjoin (e.g. boundary AB in FIG. 2, where passes 1 and 5 adjoin) is obscured by multiple overprinted swaths that have no boundary in this region (e.g. passes 2, 3, and 4).
To provide maximum flexibility for multi-pass printing, it is desirable to print text with the bottom-most nozzles on a printhead. The reason for this will be apparent from FIG. 3, which shows a line of text above a large graphic.
If the printhead (the small rectangle) is positioned at "A," then the line of text can be printed without limiting the print options for the following graphic. The multi-pass arrangement shown in FIG. 2 can be employed. If, however, the printhead is positioned at "B" to print the text, much-needed flexibility is lost for printing the graphic. This is because most printer platens cannot move the paper in reverse. The top line of the graphic will be printed with a nozzle near the top of the printhead. The shingling of FIG. 2, with different quarters of the printhead employed to print different of the successive print passes, cannot be achieved. For this reason, it is generally desirable to advance the paper as little as possible, so as to keep the lower nozzles available to start multi-pass graphics printing when needed.
Returning to the nozzle affinity problem discussed earlier, a preferred embodiment of the present invention addresses this issue by successively shifting the zone of active print nozzles up (or down) the printhead by a uniform increment in successive print swaths. By this arrangement, all nozzles are utilized, redressing the problem of idle nozzles that persists in the prior art.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.